Air purge valves are designed to be installed in liquid flow systems such as, for example, water main distribution lines or sewage collection systems, or liquid tanks, and are intended to discharge air (typically in water supply systems) or other gasses (e.g. in sewage systems), thus avoiding the formation and accumulation of gas pockets and bubbles which interfere with the liquid flow and which can also damage accessories and components of the liquid system.
A basic requirement for such gas purge valves is their ability to discharge effectively and rapidly both large and small quantities of gas whilst, at the same time, being or becoming sealed against liquid discharge. Conventional air purge valves are formed with a gas discharge outlet through which the gasses are discharged but which become sealed against liquid discharge by a float located in a valve housing and which becomes pressed against the outlet so as to seal it with a rising level of liquid in the valve housing. Such a valve, when provided with a relatively small discharge outlet, is effective for the discharge of small quantities of gas but cannot cope where large quantities have to be discharged. Where, however, in order to render the valve capable of handling large quantities of gas it is provided with a large discharge outlet (and, consequently, a large dimensioned float), problems arise in order to ensure that the valve can readily discharge rapidly relatively small quantities of gas after the discharge outlet has been sealed against liquid outflow. Thus, once the valve housing is filled with liquid and the float has been pressed against the relatively large outlet so as to seal it, the valve will only reopen once the pressure in the housing has dropped to atmospheric pressure and, in consequence, such a valve cannot be used for the continuous venting of relatively small amounts of air. Moreover, such valves have the drawback of being substantially sensitive to the presence of grit, etc., particularly where manufacturing tolerances or erosion of various components, etc., greatly influence the behavior of the float and its response to changes of the fluid flowing therethrough (liquid or gas). Thus performances of such air purge valves are somewhat inaccurate and may malfunction.
Another drawback of such air purge valves is that they tend to give rise to so-called "hammering" in the liquid system.
In particular, such gas purge valves are substantially bulky, heavy and expensive.
Proposals have also been made for gas purge valves capable of effectively and rapidly discharging both relatively small and large quantities of gas. Such a proposal is to be found in U.S. Pat. No. 4,770,201 wherein a float operated membrane is employed to open or seal successive outlet aperture regions. Such an arrangement, whilst allowing for the effective and speedy opening of the outlet aperture for speedy discharge of relatively small quantities of gas as well as its complete opening for the discharge of large quantities, is nevertheless subject to size restrictions which inherently restrict its use where large quantity discharge has to be provided for.
It is therefore an object of the present invention to provide a new and improved gas purge valve in which the above-referred-to disadvantages are substantially reduced or overcome.